Reaction bonded silicon nitride has received considerable attention in the technical literature during the last decade or so, stemming largely from work by Parr, Martin and Popper in the U.K. along with their associates, particularly those working in the British Admiralty, the Atomic Energy Establishment, and other branches of the British Government. One of the best early descriptions of the procedures for manufacturing shaped objects of silicon nitride is contained in "Special Ceramics" published by Heywood & Co., Ltd. of London in 1960, with particular reference to the article by Parr, Martin and May on pages 102 through 135. Additional details of this technology are described in U.S. Pat. No. 3,222,438 of Dec. 5, 1965, to Parr & Martin. Some of the other patents describing reaction sintered Si.sub.3 N.sub.4 and its uses are British Pat. No. 895,769, British Pat. No. 1,168,499, British Pat. No. 1,266,506, U.S. Pat. No. 2,750,268 and French Pat. No. 2,074,920.
In the proceedings of the British Ceramic Society in 1967, Volume 7, pages 81 through 98, there is an article by Parr and May which, as far as can be determined, describes the current state of the art in the manufacture of shaped parts from reaction bonded silicon nitride. Such parts are currently being offered by the British firm AME (Advanced Materials Engineering, Ltd.) utilizing, it is believed, the techniques described by Parr and May in the above 1967 article. Maximum transverse rupture strength described by Parr and May is about 36,000 psi. The material offered by AME is advertised as having transverse rupture strength on the order of 30,000 psi. This seems to be representative of other material offered by other suppliers, such as Degussa, Hoffman and Carborundum Company. None of the commercially available material is advertised as having any transverse rupture strength over 30,000 psi. In the prior art as represented by Parr and May and the other British workers, silicon powder is compressed under relatively high pressure, such as isostatic pressing, or the like, to form a compact. Prior to machining to the shape of the desired part, the prior art partially nitrides the compact to provide a "green" compact which is sufficiently strong to be handled during the machining operation. After machining, the product is then fired in a nitrogenous atmosphere to convert all of the silicon to silicon nitride. In British Pat. No. 942,082, Parr states that the mixed alpha and beta Si.sub.3 N.sub.4 produced by the above procedure can be largely converted to B Si.sub.3 N.sub.4 by continuing the heating in a nitrogen atmosphere to 1700.degree. C. As mentioned above, the maximum strength ever reported is the 36,000 psi. for transverse rupture strength (mentioned by Parr and May) while most of the state of the art is considered to be on the order of 20,000 to 30,000 psi. as measured by 3 point loading.
Evans and Davidge of A.E.R.E. Harwell have described (Journal of Material Sciences 5 (1970) 314-325) studies of the strength of reaction sintered silicon nitride wherein simple compacts (which had not been machined) were nitrided under various conditions. Maximum strengths were obtained with long nitriding of the compacts at temperatures below the 1400.degree. C. melting point of silicon. The product described by Evans and Davidge had somewhat larger particle size than that employed in the present invention and a larger final pore size than in the present invention. The stress rupture strength was on the order of 41,000 psi., (although measured with a span to thickness ratio of only 4 or 5 to 1). This strength is somewhat higher than that described by other workers who have been concerned with providing shaped parts.
U.S. Pat. No. 3,778,231 to Taylor describes a development by the Birmingham Small Arms Company (BSA) which is similar to the present invention in that the starting silicon powder compact is sintered in argon. After machining, the product is fully nitrided. The Taylor patent does not describe the characteristics of the starting silicon powder and it certainly does not mention the requisite fine particle size which is essential to achieving the results of the present invention. It seems clear that Taylor's silicon powder did not, in fact, have such a fine particle size since the reaction bonded silicon nitride offered for sale by the BSA group (according to literature which is believed to have been published some time after the issuance of the British Taylor Pat. No. 1,168,499) has a transverse rupture strength on the order of 25,000 lbs. per square inch. This is typical of the other reaction bonded silicon nitride products commercially available from other sources and is well below the results achieved by the present invention.
Flame sprayed silicon nitride, used to produce shaped parts, has been reported to have stress rupture strengths on the order of 40,000 psi. These flame sprayed parts are extremely difficult to make with any appreciable sectional thickness because of the relatively higher density of the starting silicon product and the difficulty in obtaining complete nitriding of the full cross-section.